[flang] Correct the subscripts used for arguments to character intrinsics

When chasing down another unrelated bug, I noticed that the
implementations of various character intrinsic functions assume
that the lower bounds of (some of) their arguments were 1.
This isn't necessarily the case, so I've cleaned them up, tweaked
the unit tests to exercise the fix, and regularized the allocation
pattern used for results to use SetBounds() before Allocate() rather
than the old original Descriptor::Allocate() wrapper around
CFI_allocate().

Since there were few other remaining uses of the old original
Descriptor::Allocate() wrapper, I also converted them to the
new one and deleted the old one.

Differential Revision: https://reviews.llvm.org/D104325